Errors appearing from codes of Unity Libraries

Hello, I currently see errors (they are 45 of them) inside the solution which I opened via "Open C# Project" in the Unity Project Panel. This will cause VIsual Studio 2015 to open a Solution containing the Unity Project and all the dependencies which includes libraries, Assets, Scenes and all that kind of stuff.

Since I have my Extension - Visual Studio 2015 tools for Unity installed, I can see "Attach to Unity" Button as my Run (F5) to attach my added scripts to the Unity Project. However, I see errors coming from the Unity Library itself, which of-course I didn't write or modified. There were no errors coming from my added Script and is working properly, These scripts are just for customized inspector. The Unity Console Panel is not showing errors from my Script and neither errors appearing inside the Visual Studio Solution..

Here are the first three errors..

Majority of the errors is pretty much the same (pointing out namespaces not existing or cannot be found).

Here is one instance of the remaining errors and also warnings..

Could it be a misconfiguration? An Incompatibility between Unity 5.3.3 and VS 2015 tools For Unity? Or I have just missed out something I should do before getting into my actual scripting/debugging.

I am looking forward for the help..